Triacs are semiconductor devices used to control the power taken by an electrical load from an AC line. They are found in many common applications such as light dimmers and motor speed controls for power tools. A triac is practically an open circuit until it receives a pulse of current into its "gate" terminal, whereupon it becomes practically a short circuit until the current flowing between its "main terminals" reaches zero, when it reverts to an open circuit and remains open until another pulse of gate current occurs.
If a triac is connected in series between an electrical load and an AC line, the load is connected to the line when the triac is a short circuit. During the part of the cycle when the triac is an open circuit, the load is practically disconnected from the line. If the load is resistive, the triac enters its open circuit state when the line voltage and load current are both zero. For inductive loads such as motors, current reaches zero at some determinate time after line voltage crosses zero. Therefore, for resistive or combined resistive and inductive loads, it is possible to control load power by controlling the timing of gate current pulses relative to zero crossings of the AC line.
In light dimmers or power tool speed controls, changing the gate pulse timing is done by manual adjustment of a potentiometer. There are, however, triac applications, such as automatic control of a motor, where gate pulse timing must change in response to a control voltage. The present invention is an inexpensive circuit useful for such applications. Circuits for the same purpose are known in the prior art, for example as shown in Ramshaw, R. S., "Power Electronics Semiconductor Switches", Chapman and Hall, 1993, pg. 198. However, the present invention offers significant reductions in cost and component count by using, in combination with other circuitry, a well known integrated circuit, the 555 timer, which, in a single low-cost package, has most of the functional circuit blocks necessary for triac pulse timing in response to a control voltage.
The 555 timer is described in several references which have been cited in an information disclosure statement. It is produced by many major semiconductor manufacturers in the U.S. and overseas. Each manufacturer's 555 part number is different (i.e., LM555 (National Semiconductor), MC1555 (Motorola), CA555 (Harris Semiconductor), NE555 (Texas Instrument), but all are nominally identical devices. Therefore, the term "555 timer circuit" is used to designate a circuit having the same components at the block diagram level and not the part number of the particular circuit. 555 timers are used in mass produced circuits such as refrigerator defrost cycle timers and are therefore readily available at low cost. Additionally, multiple 555 timer circuits are integrated in a single chip, such as 556 timer circuits which include two 555 timer circuits. Another advantage of the 555 timer circuit in applications of the present invention is that, being an existing device, it can be incorporated with relative ease into an application specific or custom fabricated integrated circuit in which it would be one part of an integrated control system. Still another advantage is the ability of the 555 output circuit to source or sink enough current to fire large triacs.